The invention relates to a field deflection circuit for use in a picture display device, comprising an amplifier having an input terminal for receiving a sawtooth-shaped signal of the field frequency, and an output terminal for applying a sawtooth-shaped deflection current having a trace and a retrace to a series network of a field deflection coil, a separating capacitor, and a negative feedback resistor, coupled to the said output terminal and also comprising means for feeding back the voltage across the negative feedback resistor to the amplifier.
In a field deflection circuit a d.c. coupled output amplifier, that is to say, without a separating capacitor can be used, on the condition that the amplifier has a double supply for supplying one voltage which is positive and one voltage which is negative with respect to a fixed potential, for example, ground to which the deflection coil is d.c. coupled. Such a double supply is expensive and makes it difficult to provide protection for the output amplifier.
A field deflection circuit having an a.c. coupled output amplifier, that is to say, with a capacitor for the d.c. separation of the deflection coil is therefore generally used. An example thereof has been described in German Patent Specification No. 1,462,870 (PHN.1254). Such a circuit has, however, drawbacks which the previously mentioned circuit does not have or has to a lesser extent. For example, the linearity of the deflection circuit is worse, so that correction means are to be provided in the circuit. These are, for example, negative feedback networks which must often be capable of being set and which comprise capacitors having a high capacitance. The said German patent specification also states that the input signal of the amplifier should have a parabolic component which is to be generated by a stage preceding the amplifier. Due to the frequency dependent character of the elements of the circuit, particularly of the negative feedback networks, an a.c. coupled amplifier shows a worse behaviour in the case of rapid transients in the signal to be amplified than does a d.c. coupled amplifier, which may cause a vertical instability of the picture displayed. Interlacing may also be worse because the output signal from the amplifier comprises a picture frequency component due to the non-infinitely high capacitance of the capacitor.